1. Field of the Invention
The invention relates to the field of environmental control systems for aircraft and, in particular, to a hybrid vapor cycle and air cycle environmental control system.
2. Description of Related Art
Conventional aircraft cooling system use either air cycle cooling system (ACS) or vapor cycle cooling system (VCS) to provide vehicle cooling. The vapor cycle system based on the conventional low temperature refrigerant such as R-134a is used in automobile applications may be used for low speed aircraft. The system is typically cooled by ram air but with the need for drag reduction, some systems are cooled by fuel. However, the fuel used for the cooling the system condenser has to be limited in a relative low temperature level such as lower than 140 degree F. The VCS has a higher coefficient of performance (COP) because the high level of latent heat change from liquid to vapor state. The COP is defined as the cooling capacity versus the work required by the system.
The air cycle cooling system in various configurations has been used for most of aircraft cooling design. Most ACS use ram air to cool the engine bleed air using a primary heat exchanger before it is compressed and then cooled by a ram air-cooled secondary heat exchanger after the air is compressed by a compressor. The cooled air then is expanded through the cooling turbine to produce low temperature cooling air for distribution in to the aircraft for equipment, subsystem, and cockpit. For aircraft designed for high. Mach number cruise, the conventional system uses fuel to cool the engine bleed air instead of using ram air because the ram air total temperature could be high and the ram air inlet increases drag. Significant safety pre-cautions have to be taken to mitigate the catastrophic failure that may occur if fuel leakage comes in contact with high temperature air. The ACS has a lower COP because of the high power required to compress air.
For high-speed flight, hybrid system conbining ACS and VCS has been used for ram air drag reduction and fuel heat sink temperature matching. In the prior art, the heat rejection from equipment is first sank into the VCS and then the heat rejection from VCS is transferred by the ACS and then dissipated into fuel and ram air. Because the ACS is inherently low in efficiency and it has to be designed to dissipate the cooling load and the work required by the VCS, there is a weight penalty. However, this approach is used because the VCS is more efficient in producing low temperatures using the phase change property of the refrigerant.
With advent of high power electronics, the cooling loads are greatly increased, which exacerbates the problem. The bleed air that can be extracted from the engine for high speed-high altitude flight is relatively low; otherwise engine performance is significantly reduced. For an ACS, primarily using bleed air, becomes a less attractive solution. In addition, with high-speed cruise at Mach 2+, the aerodynamic heating contributed to the fuel tank temperature increases significantly and the ram air total temperature is high. The system design described herein addressed these issues using unique integration and cooling loads partitioning approach.
Thus, it is a primary object of the. invention to provide an environmental control system for an aircraft.
It is another primary object of the invention to provide an environmental control system for an aircraft that is suitable for use in aircraft flying at very high velocities and high altitude.
It is a further object of the invention to provide an environmental control system for an aircraft that is a hybrid air cycle system and vapor cycle system.